EPOXI: Clues to Terrestrial Worlds

byPaul GilsteronJuly 18, 2008

You must see new video from EPOXI, whose effect can only be suggested by the photo montage below (click the link below for the movie). EPOXI is the combined extended mission of the Deep Impact spacecraft. As we discussed in an earlier story here, EPOXI turned its cameras on the Earth to view the moon transiting the planet’s disk from a vantage point of 31 million miles. Think in terms of viewing the Earth the way we will eventually view terrestrial worlds around other stars. The idea is to build insights into how these worlds can be observed and characterized.

Drake Deming (NASA GSFC), deputy principal investigator for EPOXI and leader of the extrasolar planet component of the mission (called EPOCh), points out how the information can be helpful:

“Our video shows some specific features that are important for observations of Earth-like planets orbiting other stars. A ‘sun glint’ can be seen in the movie, caused by light reflected from Earth’s oceans, and similar glints to be observed from extrasolar planets could indicate alien oceans. Also, we used infrared light instead of the normal red light to make the color composite images, and that makes the land masses much more visible.”

None of our current or projected planet hunter missions will be able to see a terrestrial world as close up as this, but we will be able to see such a planet as a point of light that can be studied to observe changes in its total brightness. Such changes could be telling us about continents on a planet affecting the light signature as, interspersed with water, they rotate in and out of view. So taking views of a known living world helps us understand how these light changes occur, and we’ll want to keep taking pictures of Earth at ever greater distances to continue that effort. The moon transit occurring in these frames is a priceless bonus.

Now take a look at the second movie. Whereas the first was made using a red-green-blue filter, the second was made using infrared-green-blue. With plants reflecting more strongly in the near-infrared, the land masses become more readily visible. We’re thus looking at the possibility of detecting vegetation on extrasolar planets whose marker will be changes in near-infrared light as the planet rotates. All this in the future from a tiny point of light whose properties we’re learning to parse thanks to a mission that once set out to drive an impactor into a comet.

Yes, the transiting satellite does affect the measurements, but this is a good thing here because it allows scientists to look at still another way the light signature may vary. As to EPOXI, it’s a terrific mission so far, and I have a hunch the EPOCh component that Drake Deming is running is going to have still more useful things to tell us down the road.

Occultation can be an incredibly powerful tool in low-resolution observations. For example, during a period of numerous mutual eclipses of Pluto and Charon many observations of the system were performed and used to create albedo maps of Pluto and Charon. Sometimes it’s amazing what sort of data you can extract from limited measurements. The detection of extra-solar planets through spectrographic study is an excellent example of just that.

There’s something deeply humbling about seeing the enormous thing we live on and almost all of us have lived on all our lives and our ancestors’ for billions of years, shrunk down and pixellated from a camera 31 million miles away.

Just a thought. The Moon came up pretty easily, and I suppose it wouldn’t be impossible that we’d find other terrestrial planets with moons – after all, we have one and Mars has two, so it can’t be vanishingly unlikely. What is the probability that you’d find a binary planet which is habitable?

Benjamin, I had the same reaction when looking at the EPOXI video. ‘Humbling’ is exactly the right word. The situation with the Moon is a bit tricky. It’s now thought that the Moon came about because of a collision between the early Earth and a Mars-sized object. How common would such collisions be? Given current models of planetary formation, I would assume they wouldn’t be totally unusual — there were lots of collisions in the early circumstellar disk — but I have no idea what the odds would be. Maybe some of the readers have seen work on such calculations. By ‘a binary planet which is habitable,’ I assume you’re talking about two bodies of roughly the same size (Pluto and Charon would be the closest we have to this in our Solar System), and in the habitable zone. Here again I have seen no calculated probabilities, but maybe someone else can give us a pointer.

We will analyze these data for evidence of additional planets, via transit timing variations or transits; for planetary moons or rings; for detection of secondary eclipses and the constraint of geometric planetary albedos; and for refinement of the system parameters.

Over a period of four months, EPOCh observed four known transiting planet systems, with each system observed continuously for several weeks. Here we present an overview of EPOCh, including the spacecraft and science goals, and preliminary photometry results.

Abstract: The equilibrium rotation of tidally evolved “Earth-like” extra-solar planets is often assumed to be synchronous with their orbital mean motion. The same assumption persisted for Mercury and Venus until radar observations revealed their true spin rates.

As many of these planets follow eccentric orbits and are believed to host dense atmospheres, we expect the equilibrium rotation to differ from the synchronous motion.

Here we provide a general description of the allowed final equilibrium rotation states of these planets, and apply this to already discovered cases in which the mass is lower than twelve Earth-masses. At low obliquity and moderate eccentricity, it is shown that there are at most four distinct equilibrium possibilities, one of which can be retrograde. Because most presently known “Earth-like” planets present eccentric orbits, their equilibrium rotation is unlikely to be synchronous.

Comments: 4 pages, 2 figures. accepted for publication in Astronomy and Astrophysics. to be published in Astronomy and Astrophysics

By looking back at Earth from alien’s point of view, scientists have developed a new technique to look for other worlds that might harbor oceans, and therefore life.

Using the old Deep Impact spacecraft, which is now being used for the EPOXI mission, scientists are able to look at the spectrum of an extrasolar planet’s light which would reveal the presence of water.

“We used the High Resolution Imager telescope on Deep Impact to look at Earth from tens of millions of miles away,” said Nicolas B. Cowan, of the University of Washington, “and developed a method to indicate the presence of oceans by analyzing how Earth’s light changes as the planet rotates. This method can be used to identify extrasolar ocean-bearing Earths.”

Last year, the EPOXI science team was able to take videos of the Moon transiting Earth, (see our article from July 2008). The team has now practiced the technique by looking back at Earth, and have determined that they should be able to detect oceans on other worlds by looking at the changing spectrum of light the planet gives off as it rotates.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last eleven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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